FIG. 6 is a sectional view illustrating a conventional n-p-n type GaAs HBT. In the figure, reference numeral 51 designates a semi-insulating GaAs substrate. An n.sup.+ type GaAs collector contact layer 52 is disposed on the GaAs substrate 51. An n type GaAs collector layer 53 is disposed on a part of the collector contact layer 52. A p.sup.+ type GaAs base layer 54 is disposed on the collector layer 53. An n type Al.sub.0.3 Ga.sub.0.7 As emitter layer 55 is disposed on a part of the base layer 54. An emitter electrode 58 is disposed on the emitter layer 55. Base electrodes 57 are disposed on the base layer 54 at opposite sides of the emitter layer 55. Collector electrodes 56 are disposed on the collector contact layer 52 at opposite sides of the collector layer 53. Reference numeral 59 designates insulating regions formed by ion implantation.
FIG. 7 is an energy band diagram of the HBT of FIG. 6 during operation. In the figure, the same reference numerals as in FIG. 6 designate the same parts.
A description is given of the operation. In FIG. 7, a bias voltage V.sub.BE is applied between the emitter layer 55 and the base layer 54, and a bias voltage V.sub.CB is applied between the base layer 54 and the collector layer 53. Electrons flow from the emitter layer 55 through the base layer 54 to the collector layer 53. Holes flow from the base layer 54 to the emitter layer 55. In this HBT, the bias voltage V.sub.BE controls the flow of electrons and holes.
In this conventional n-p-n GaAs HBT, since the collector layer 53 usually comprises n type GaAs, when the bias voltage V.sub.CB is applied between the base layer 54 and the collector layer 53 during operation, a strong electric field is applied to the interface between the base layer and the collector layer. The electric field causes intervalley scattering of electrons, whereby the electron mobility is reduced and the collector transit time is increased. As a result, the operating speed of the HBT is reduced.